1. Field of the Invention
The present invention relates to radio communication technology, and more particularly to a handoff control technique in a mobile communication system wherein a frequency for a radio communication with an access terminal differs every base station.
2. Description of the Related Art
Connections from mobile terminals to the Internet have been more used year by year, and needs have been heightened, not only for browsing Web pages, but also for transmitting/receiving music and images and the like data of large capacities. In order to realize high-speed Web accesses and high-speed data communications in radio communications under such a background, studies for heightening a communication speed and widening a communication band have been rapidly promoted. In a radio communication system employing the present-day “1xEV-DO Rev. A” scheme, communications in a reverse capacity of 1.8 Mbps and a forward capacity of 3.1 Mbps are possible. In a system of the next generation, studies have been promoted for the realization of the system which, aiming at a still higher communication speed and a still wider communication band, permits communications in a reverse capacity of at least 50 Mbps and a forward capacity of at least 100 Mbps.
On the other hand, radio wave resources are finite, and the insufficient band of radio waves becomes a matter due to the increase of the number of users and the heightened speed of the communications. Also, a frequency band which each individual base station can process is definite. Therefore, when a large number of users attempt to communicate in the area of one base station, the frequency band of the radio waves becomes insufficient, and the congestion of the communications and the lowering of the data communication speed take place. In order to solve the matter, a common carrier intends to eliminate the frequency insufficiency by a method such as increasing the number of the base stations, in a city region where the users concentrate in large numbers.
Besides, a very small-sized base station called “femtocell” has recently been studied. The femtocell is a system which is configured so as to be connectable to a broadband channel by reducing the size of the base station of a portable telephone. The base stations of the femtocells are installed in underground and indoor areas into which the radio waves are difficult to enter, whereby even the areas and complicated indoor parts at which the radio waves have heretofore been difficult to arrive can be covered. In a radio communication system which is configured having the base stations of the femtocells, a countermeasure against the matter of the frequency insufficiency in the base stations is entrusted to a user side.
The femtocells which are the base stations of new frequencies are installed anew and are utilized, whereby the base stations of a plurality of different frequencies coexist in the radio communication system. In such a system in which the base stations of the different frequencies coexist, when the access terminal moves between the base stations of the different frequencies, the base station with which the access terminal communicates needs to be switched to the base station of the different frequency. This operation is called “inter-frequency handoff”. In contrast, handoff in the case where the access terminal moves between the base stations which use an identical frequency is called “soft handoff”.
The inter-frequency handoff includes a method of terminal-led type and a method of base station-led type. A typical example of the terminal-led type is Mobile Assist Handoff (MAHO). In the MAHO, the access terminal is always receiving the information items of neighboring base stations (the neighboring base stations here include both the base stations of the identical frequency and the different frequency) from the base station with which this access terminal is currently communicating. In the neighboring base station information items, PN information items (the identification codes of respective sectors in a CDMA system) and the frequency information items are contained as the information items on the base station neighboring the base station with which the access terminal is currently communicating.
The access terminal receives pilot signals which are successively sent in by the control channels of the respective base stations, and it reports received power values to the base station (Serving Base Station: Serving BS) with which it is currently communicating. Usually, the access terminal searches for only a neighbor pilot which is the pilot signal of the neighboring base station of the identical frequency, but it begins a search for the neighbor pilot of the neighboring base station of the different frequency, at the very limit outside the communication area. In a case where the reception power value of the neighbor pilot has exceeded a threshold value, the access terminal notifies to that effect to the base station with which it is currently communicating, by a route update message. The base station transmits to the access terminal a Traffic Channel Assignment message by which the pilot is made an active set.
On the other hand, a typical example of the base station-led type is a handoff method which uses a pilot beacon. The “pilot beacon” is a pilot signal which the neighboring base station of the frequency different from that of the serving base station transmits to the access terminal by using the same frequency as that of the serving base station. The access terminal reports the pilot reception power value from the serving base station and a reception power value from the pilot beacon, to a Base Station Controller (abbreviated to “BSC”). Besides, in a case where the reception power value from the pilot beacon has exceeded a threshold value, the BSC commands the access terminal to hand off to the neighboring base station of the different frequency.
Although the inter-frequency handoff is not concerned, Patent Document 1 (JP-A-2002-232928) and Patent Document 2 (JP-A-9-261711) disclose inventions concerning the selection of a handoff destination. Patent Document 1 proposes a soft handoff control method which uses the GPS positional information items of an access terminal, with the object of preventing any erroneous handoff ascribable to the instability of a reception power value in a multi-path environment.
Besides, in Patent Document 2, with the object of performing a search for the handoff destination efficiently in a short time, the optimum base station is decided as the handoff destination on the basis of the positional information items of an access terminal as received from a GPS for the access terminal and a database which is retained in a base station controller and in which information items on the set positions of individual base stations are stored beforehand.